The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour i...The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour is analyzed for deformation temperatures between RT and 950℃, Fracture resistance behaviour and toughening mechanisms at RT and 800℃ are analyzed. and the inverse relationship botween ductility and toughness is explained using the crack initiation toughness. The preliminary results of load-controlled fatigue behaviour at 800℃ are interpreted using the tensile behaviour because deformation structure and fracture modes are similar under these two loading conditions展开更多
This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombi...This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.展开更多
This paper describes a new process for producing titanium aluminides, in particular TiAl, from TiO2 raw material. On the basis of obtained results, the non-completed reaction of TiO2 with Al and Ca in a special reacti...This paper describes a new process for producing titanium aluminides, in particular TiAl, from TiO2 raw material. On the basis of obtained results, the non-completed reaction of TiO2 with Al and Ca in a special reaction vessel results in the production of granulates of titanium aluminides especially Ti3Al and other Ti- Al phases as the metallic product and Ca12Al14O33 as the non-metallic product. By adding KClO4 in the mixture, a nearly completed reaction can be carried out. The products of this reaction are titanium aluminide particularly TiAl as the metallic part and CaAl4O7 （grossite） as the non-metallic slag part. Both product and slag are produced in a separated form. This process, called KRH-method is described in this article. The scanning electron microscopic microstructure of metallic part of the product shows different phases： the matrix phase is TiAI, where the needle form precipitation is TiAl2 and the plate form precipitation includes TiAI and Ti3Al phases. The microstructure of the remelted metallic part indicates dendritic phase with a lamellar structure comprising of TiAl and Ti3Al phases. The interdendritic phase of TiAI is also seen.展开更多
Twinning is an important deformation mechanism in engineering materials,which can improve the ductility and strength of alloys,especially in the case of low-plasticity Ti Al alloys.Herein,a large number ofγtwins,twin...Twinning is an important deformation mechanism in engineering materials,which can improve the ductility and strength of alloys,especially in the case of low-plasticity Ti Al alloys.Herein,a large number ofγtwins,twin intersections and adjoining twin pairs(ATPs)are formed in globularγ(γ-glob)phase after high-temperature tensile deformation.The ATPs consist of two sets of deformation twins(DTs)that are mirror symmetric about the original twin boundary(OTB).Its formation mechanism is analyzed with electron back-scattered diffraction(EBSD)and transmission electron microscope(TEM).The results reveal that the ATPs are related to the strain coordination between the DTs and the adjacent grains.Specifically,as the DT is formed in theγ-glob and impinges on the OTB,it will induce anotherγtwin(IT)in the adjacentγ-glob.The combination of the twoγtwins generates ATPs,which are formed by the transmission of a twin from one grain to the neighboring grain.Moreover,γtwins with a lower Schmidt factor(SF)of 0.2643 are preferentially formed over anotherγtwin with a higher SF(0.3098)in the sameγ-glob grain,becauseγIT accommodates the strain caused by theγDT in adjacent grain.Besides,the long-period stacking ordering(LPSO)structure of periodic contrasts 1(PC_(1))and PC_(2)are observed inγtwins,which are considered to be the intermediate phase for the formation ofγtwins.The current work elucidates the formation mechanism of ATPs in Ti Al alloy and provides insights into the formation of twins in theγ-glob.展开更多
The microstructure development of lamellar structure of an orthorhombic Ti2AlNb-based Ti？22Al？26Nb？1Zr alloy, includingB2 decomposition and spheroidization ofO phase, was investigated. The results show that the lam...The microstructure development of lamellar structure of an orthorhombic Ti2AlNb-based Ti？22Al？26Nb？1Zr alloy, includingB2 decomposition and spheroidization ofO phase, was investigated. The results show that the lamellar structure is fabricated by heating the samples in the singleB2 phase field and cooling slowly in the furnace. Aging treatments are conducted in the （O＋B2） phases field by air cooling. After aging at 700 °C for a short time within 100 h, there is no significant change of microstructures, whereas the coarsening of lamellae is observed in the long-term aged microstructure. Ti？22Al？26Nb？1Zr alloy exhibits microstructural instability including the severe dissolution ofB2 lamella, discontinuous precipitation and spheroidization of O phase during the long term aging process at 700 °C up to 800 h. In addition, a pronounced formation of branch-shapedO phase lamella is observed for the alloy aged over 100 h.展开更多
Pure titanium samples were aluminized at 950,1025 and 1100 ℃ for 0-6 h in a pack containing 10%Al＋5%NaF＋85%Al2O3 in mass traction.The aluminized layers formed on the samples were characterized.The kinetic studies i...Pure titanium samples were aluminized at 950,1025 and 1100 ℃ for 0-6 h in a pack containing 10%Al＋5%NaF＋85%Al2O3 in mass traction.The aluminized layers formed on the samples were characterized.The kinetic studies indicated that the diffusion of Al-bearing gases through the pack is the rate-controlling step in this process.The activation energy of 161.8 kJ/mol was calculated for this step.In addition,the mass gains of the aluminized samples were predicted using the partial pressures of gases in the pack and those adjacent to the samples surface.The predicted values are in good agreement wim the experimental measurements at 950 ℃ but are higher than those measured at 1025 and 1100 ℃.展开更多
The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed ...The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed in welds of titanium alloy laser welding based on the microscopic characteristics of the porosities. One is the metallurgical porosity with round and smooth inner wall, which results from the surface contamination. The other is the processing porosity with irregular and rough inner wall that displays the trace of the pool flowing, which results from the ruffle on the keyhole wall gathering together locally and closing down the gas in the keyhole into bubbles because of the keyhole fluctuating. The CO2 laser welding could break down easily the surface oxide film and produce little metallurgical porosity, but produces easily processing porosity when partial penetration or unstable-full penetration laser welding is conducted, which always occurs in the center of weld.展开更多
Gam matitanium aluminide ( γ TiAl) alloys are emerging as a revolutionary engineeringmaterialsfor hightemperaturestructuralapplications. Onthebasisoftheinformation avail ablein the public domain, this paper discuss...Gam matitanium aluminide ( γ TiAl) alloys are emerging as a revolutionary engineeringmaterialsfor hightemperaturestructuralapplications. Onthebasisoftheinformation avail ablein the public domain, this paper discussesthe historical background,status and future prospect of gam maalloytechnologyintheareasofalloy development/ design,processdevelop ment, and applications.展开更多
Reaction synthesis process has been used to develop γ titanium aluminide using elemental powders. Powder mixture of Ti-48 at. pct AI was prepared in ball mill and reaction synthesis was carried out in hot press with ...Reaction synthesis process has been used to develop γ titanium aluminide using elemental powders. Powder mixture of Ti-48 at. pct AI was prepared in ball mill and reaction synthesis was carried out in hot press with varying temperature and pressure. Titanium aluminide synthesized under high pressure and temperature resulted in better properties with respect to densification, homogenization response, mechanical properties and oxidation resistance as compared to that synthesized under low pressure and temperature. AI rich phases were observed in as-synthesized condition in all the experiments. However, some Ti rich phases were also found in high pressure-temperature synthesized samples. Density, hardness and tensile strength have been correlated with applied pressure through empirical relations. Variation in density with pressure is found to be logarithmic whereas hardness and tensile strength variation with pressure is polynomial.展开更多
The relationship between the stress intensity factor K_1~* required for brittle crack initiation and propa- gation and the fractal dimension D_F of the fracture surface was derived,i.e, InK_1~*=(1/2)In2γE'+(1/2)I...The relationship between the stress intensity factor K_1~* required for brittle crack initiation and propa- gation and the fractal dimension D_F of the fracture surface was derived,i.e, InK_1~*=(1/2)In2γE'+(1/2)In(d_f/L_0)(1-D_F) The real surface energy can be calculated based on the measured linear relation of the InK_1~* vs D_F, The equation is not only suitable for the overload fracture but also for the delayed fracture,e.g. hydrogen induced cracking (HIC) and stress corrosion cracking (SCC).The experiment results showed that the hydrogen induced delayed cracking occurred in the Ti-24AI-11 Nb alloy during dy- namic charging,and the threshold stress intensity factor was very low,i.e..K_(IH)/K_(IC)= 0.43.The ex- perimental relationship between the stress intensity factor K_1~* and D_F was consistent with the theo- retical equation.展开更多
This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique.The results...This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique.The results indicate that the mushy zone consists of unmeltedβdendrites and interdendritic liquid,whose formation can be attributed to the difference in melting point aroused by local heterogeneity in solutecontent.Theβdendrite is composed of numerous subgrains with various orientations.During quenching,theβdendrite transforms into Widmanstättenαvia a precipitation reaction,owing to the decreasing cooling rate caused by heat transfer from the surrounding liquid.Additionally,after quenching,the interdendritic liquid is transformed intoγplates.Within the singleβphase field and the lower part of the mushy zone,a massive transformation ofβtoγoccurs.Conversely,in theβ+αphase field,bothβandαphases are retained to ambient temperature.During the heating process,the transformation ofα→βgives rise to the formation ofβvariants,which affects the orientation ofβdendrites in the mushy zone.The growth kinematics of theα→βtransformation was elucidated,revealing the preferential growth directions of111and112forβvariants.Furthermore,this study presents an illustration of the formation process of the mushy zone and the microstructural evolution during the heating and quenching process.展开更多
The double directional solidification（DS） technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys.Polysynthetically twinned（PST） crystals with lamellar boundaries parallel to o...The double directional solidification（DS） technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys.Polysynthetically twinned（PST） crystals with lamellar boundaries parallel to or inclined 45o to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface.The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions.Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase.Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys,the tensile plastic elongations of the alloys are only close to 2%.展开更多
The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%)alloy after subsequent quenching from 1380℃to 1000℃was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-bac...The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%)alloy after subsequent quenching from 1380℃to 1000℃was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM)and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→βo+α2+γ→βo+γ+α2/γ→βo+γ+α2/γ+βo,sec,with the phase transformationα→βtemperature(Tβ)=1311℃,phase transformationγ→βtemperature of(Tγsolv)=1231℃,phase transformationα2→αorβo→βtemperature(Tα2→α/Tβo→β)=1168 C,eutectoid temperature(Teut)=1132℃and T(α2/γ→βo,sec)≈1120℃.In comparison with Ti-42 Al alloy,the Teut and Tγsolv are slightly increased while both the Tp is decreased obviously by 5%Mn addition.When quenched from the temperature of 1380-1260℃,the martensitic transformationβ→α'could occur to form the needlelike martensite structure inβarea.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃to 1380℃.When the temperature is below Tγsolv(1231℃),theγgrains would nucleate directly from theβphase.For the temperature slightly lower than T(eut)(1132℃),the dottedβ(o,sec)phases could nucleate in the lamellar colonies besides theγlamellae precipitated withinα2 phase.Finally,at room-temperature(RT),the alloy exhibits(po+α2+γ)triple phase with microstructure ofβo+lamellae+γ,of which the lamellar structure consists ofα2,γandβo,sec phases.The phase transformation mechanisms in this alloy,involvingβ→α',β→γ,α2→α2/γandα2→βo,sec were discussed.展开更多
Effects of cooling rate and 0.25 at.%TiB2 addition on solidification microstructure and mechanical properties of Ti−48Al−2Cr−2Nb alloys fabricated by the investment casting with different thicknesses were studied.The ...Effects of cooling rate and 0.25 at.%TiB2 addition on solidification microstructure and mechanical properties of Ti−48Al−2Cr−2Nb alloys fabricated by the investment casting with different thicknesses were studied.The results show that with the cooling rate increasing from 37 to 2×102 K/s,the solidification path of the studied alloys is unchanged.The grain size of the matrix alloy is refined from 650 to 300μm,while the grain size of Ti−48Al−2Cr−2Nb−TiB2 is reduced from 550 to 80μm.The lamellar spacing of matrix alloy is reduced from 360 to 30 nm with increasing the cooling rate from 37 to 2×102 K/s,while TiB2 addition shows little refinement effect on the lamellar spacing.Ti−48Al−2Cr−2Nb−TiB2 sample under medium cooling rate(69 K/s)exhibits superior microhardness(HV 550)and ultimate tensile strength(570 MPa)among the studied alloys.The refined grain size,lamellar spacing and fine TiB2 particles could account for the favorable mechanical properties of the studied TiB2-containing alloy.The microstructure evolution was discussed in light of cooling rate,constitutional supercooling and borides addition.展开更多
Solid state phase transformation characteristics of the body centred β（Ti） into the hexagonal closed packed α（Ti） in Ti45AI8Nb-（0, 0.3, 0.5 and 0.8 at. pct） B alloys were investigated by heat treatment to clar...Solid state phase transformation characteristics of the body centred β（Ti） into the hexagonal closed packed α（Ti） in Ti45AI8Nb-（0, 0.3, 0.5 and 0.8 at. pct） B alloys were investigated by heat treatment to clarify γ/α2 lamellar microstructure refinement induced by B addition. Experimental results confirmed two kinds of boron-reduced grain refinement mechanisms through refining either/β phase then α phase （β-refinement） or α phase directly （α-refinement） to refine lamellar microstructure at room temperature; however, the role of α-refinement dominated the as-cast alloy. It was also found that during the lamellar microstructure refinement β-refinement the convoluted flake over β-refinement in Ti45Al8NbxB and plate-like borides along β grain boundaries assisted nucleation of α phase, and the particle-like borides nearβ grain boundaries impeded α phase growth.展开更多
Microstructures and deformation properties of Ti-46Al-(Cr,Nb,W,B)alloy consolidated by pseudo-HIP technology were investigated.The results show that the pseudo-HIP temperature has a significant effect on microstructur...Microstructures and deformation properties of Ti-46Al-(Cr,Nb,W,B)alloy consolidated by pseudo-HIP technology were investigated.The results show that the pseudo-HIP temperature has a significant effect on microstructures.When the sintering temperature is 1 100℃,the microstructure of as-pseudo-HIPped alloy is similar to that of the prealloyed powder and the interfaces of these powder particles are still discernible,but a nearγmicrostructure appears in particles.Increasing the pressing temperature to 1 200℃develops successfully a homogeneous and fine-grained duplex microstructure.A typically fully lamellar microstructure with residualβphase is developed at 1 300℃.The compact exhibits excellent deformation properties at elevated temperatures. When the compression temperature is higher than 1 100℃,high quality products without cracks can be obtained even if the engineering compression strain is up to 0.8 at strain rates of 10-2-10-3s-1.It can be established that the mechanical twinning and matrix deformation due to ordinary dislocation slip/climb contribute to the whole hot deformation.展开更多
A 0.7 mm-thick wavy γ-TiAl sheet with fully lamellar microstructure was fabricated by hot pressing Ti/Al alternate foils with heat treatment of 640 °C, 15 h＋850 °C, 35 h＋1350 °C, 2 h. The intermetall...A 0.7 mm-thick wavy γ-TiAl sheet with fully lamellar microstructure was fabricated by hot pressing Ti/Al alternate foils with heat treatment of 640 °C, 15 h＋850 °C, 35 h＋1350 °C, 2 h. The intermetallic compounds formed during heat treatments were identified by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. TiAl3 was the only observed phase at the Ti/Al interface when Al foils were not consumed. After being annealed at 850 °C for 35 h, the microstructure was composed of α-Ti, α2-Ti3Al, γ-TiAl and TiAl2. A fully lamellar microstructure formed after annealing at 1350 °C. Most of the angles between the lamellar interface and the sheet plane are below 30°. Using thinner starting foils is favorable to produce sheets with fine microstructure.展开更多
A γ-TiAI intermetallic alloy, Ti-45Al-2Nb-2Mn （at.%）-0.8 vol.%TiB2, has been processed from gas atomized praalloyed powder by field assisted hot pressing （FAHP）. An initial analysis of the prealloyed powder helpe...A γ-TiAI intermetallic alloy, Ti-45Al-2Nb-2Mn （at.%）-0.8 vol.%TiB2, has been processed from gas atomized praalloyed powder by field assisted hot pressing （FAHP）. An initial analysis of the prealloyed powder helped on the understanding of the intermetallic sintering behavior. Atomized powder consisted of α metastable phase that transformed into α2＋γ equilibrium phases by thermal treating. Different powder particle microstructures were found, which influence the microstructure development of the FAHP T-TiAI material depending on the sintering temperature. Duplex, nearly lamellar and fully lamellar microstructures were obtained at the sintaring temperatures above 1000 ℃. Lower consolidation temperatures, below 1000 ℃, led to the formation of an AI rich phase at powder particle boundaries, which is deleterious to the mechanical properties. High compressive yield strength of 1050 MPa was observed in samples with FAHP duplex microstructures at room temperature. Whereas nearly lamellar and fully lamellar microstructures showed yield strength values of 655 and 626 MPa at room temperature and 440 and 425 MPa at 750 ℃, respectively, which are superior in comparison to similar alloys processed by other techniques. These excellent properties can be explained due to the different volume fractions of the α2 and γ phases and the refinement of the PM microstructures.展开更多
The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100℃was systematically investigated in the temperature range of 750-900℃under an initial strain rate of 10^(-4)s^(-1).A bimodalγgraindistribution mi...The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100℃was systematically investigated in the temperature range of 750-900℃under an initial strain rate of 10^(-4)s^(-1).A bimodalγgraindistribution microstructure of Ti Al alloy sheet,with abundant nano-scale or sub-micronγlaths embedded insideβmatrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m=0.27 at 800℃,which is the lowest temperature of superplastic deformation for Ti Al alloys attained so far.The maximum elongation reaches~360%at 900℃with an initial strain rate of 2.0×10^(-4)s^(-1).To elucidate the softening mechanism of the disorderedβphase during superplastic deformation,the changes of phase composition were investigated up to 1000℃using in situ high-temperature X-ray diffraction(XRD)in this study.The results indicate thatβphase does not undergo the transformation from an ordered L2;structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of bothγandβdynamic recrystallization(DR)after tensile tests as characterized with electron backscatter diffraction(EBSD),the superplastic deformation mechanism can be explained by the combination of DR and grain boundary slipping(GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic deformation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic Ti Al alloy.展开更多
In order to understand the effect of Mo alloying on the microstructural evolution of TiAl alloy, the as-cast microstructure, heat treated microstructure characteristic, and hot compression microstructure evolution of ...In order to understand the effect of Mo alloying on the microstructural evolution of TiAl alloy, the as-cast microstructure, heat treated microstructure characteristic, and hot compression microstructure evolution of Ti-44Al alloy have been studied in this work. The as-cast microstructure morphology changes from （γ＋α2） lamellar colony and β/β0＋γ mixture structure to β/β0 phase matrix widmannstatten structure, when Mo content increases from 2 at.% to 6 at.%. Affected by the relationship between β phase and α90℃phase, the angles between the lamellar orientation and the block β/β0 phase are roughly at 0℃, 45℃ and 90℃. Comparing with heat treatment microstructure, the hot compression microstructure contains less β/β0 phase, however, theβ/β0 phase containing 2Mo alloy and 3Mo alloy hot compressed at 1275 ℃ has the inverse tendency. In addition, （α2 ＋γ） colony is decomposed by the discontinuous transformation.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.展开更多
文摘The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour is analyzed for deformation temperatures between RT and 950℃, Fracture resistance behaviour and toughening mechanisms at RT and 800℃ are analyzed. and the inverse relationship botween ductility and toughness is explained using the crack initiation toughness. The preliminary results of load-controlled fatigue behaviour at 800℃ are interpreted using the tensile behaviour because deformation structure and fracture modes are similar under these two loading conditions
基金funded by National Project PCCA contract No.65/2012POS-CCE O 2.2.1 project INFRANANOCHEM-No 19/January 3,2009 of the EU(ERDF)and Romanian Government
文摘This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.
文摘This paper describes a new process for producing titanium aluminides, in particular TiAl, from TiO2 raw material. On the basis of obtained results, the non-completed reaction of TiO2 with Al and Ca in a special reaction vessel results in the production of granulates of titanium aluminides especially Ti3Al and other Ti- Al phases as the metallic product and Ca12Al14O33 as the non-metallic product. By adding KClO4 in the mixture, a nearly completed reaction can be carried out. The products of this reaction are titanium aluminide particularly TiAl as the metallic part and CaAl4O7 （grossite） as the non-metallic slag part. Both product and slag are produced in a separated form. This process, called KRH-method is described in this article. The scanning electron microscopic microstructure of metallic part of the product shows different phases： the matrix phase is TiAI, where the needle form precipitation is TiAl2 and the plate form precipitation includes TiAI and Ti3Al phases. The microstructure of the remelted metallic part indicates dendritic phase with a lamellar structure comprising of TiAl and Ti3Al phases. The interdendritic phase of TiAI is also seen.
基金financially supported by the National Natural Science Foundation of China(No.52174377)the China Postdoctoral Science Foundation(No.GZC20240524)+2 种基金the China Postdoctoral Science Foundation(No.2024M750998)the National Key Research and Development Program of China(No.2021YFB3702603)the National Natural Science Foundation of China(NSFC)(No.51371049)。
文摘Twinning is an important deformation mechanism in engineering materials,which can improve the ductility and strength of alloys,especially in the case of low-plasticity Ti Al alloys.Herein,a large number ofγtwins,twin intersections and adjoining twin pairs(ATPs)are formed in globularγ(γ-glob)phase after high-temperature tensile deformation.The ATPs consist of two sets of deformation twins(DTs)that are mirror symmetric about the original twin boundary(OTB).Its formation mechanism is analyzed with electron back-scattered diffraction(EBSD)and transmission electron microscope(TEM).The results reveal that the ATPs are related to the strain coordination between the DTs and the adjacent grains.Specifically,as the DT is formed in theγ-glob and impinges on the OTB,it will induce anotherγtwin(IT)in the adjacentγ-glob.The combination of the twoγtwins generates ATPs,which are formed by the transmission of a twin from one grain to the neighboring grain.Moreover,γtwins with a lower Schmidt factor(SF)of 0.2643 are preferentially formed over anotherγtwin with a higher SF(0.3098)in the sameγ-glob grain,becauseγIT accommodates the strain caused by theγDT in adjacent grain.Besides,the long-period stacking ordering(LPSO)structure of periodic contrasts 1(PC_(1))and PC_(2)are observed inγtwins,which are considered to be the intermediate phase for the formation ofγtwins.The current work elucidates the formation mechanism of ATPs in Ti Al alloy and provides insights into the formation of twins in theγ-glob.
基金Project(2011CB605503)supported by the National Basic Research Program of ChinaProject(51371144)supported by the National Natural Science Foundation of China
文摘The microstructure development of lamellar structure of an orthorhombic Ti2AlNb-based Ti？22Al？26Nb？1Zr alloy, includingB2 decomposition and spheroidization ofO phase, was investigated. The results show that the lamellar structure is fabricated by heating the samples in the singleB2 phase field and cooling slowly in the furnace. Aging treatments are conducted in the （O＋B2） phases field by air cooling. After aging at 700 °C for a short time within 100 h, there is no significant change of microstructures, whereas the coarsening of lamellae is observed in the long-term aged microstructure. Ti？22Al？26Nb？1Zr alloy exhibits microstructural instability including the severe dissolution ofB2 lamella, discontinuous precipitation and spheroidization of O phase during the long term aging process at 700 °C up to 800 h. In addition, a pronounced formation of branch-shapedO phase lamella is observed for the alloy aged over 100 h.
文摘Pure titanium samples were aluminized at 950,1025 and 1100 ℃ for 0-6 h in a pack containing 10%Al＋5%NaF＋85%Al2O3 in mass traction.The aluminized layers formed on the samples were characterized.The kinetic studies indicated that the diffusion of Al-bearing gases through the pack is the rate-controlling step in this process.The activation energy of 161.8 kJ/mol was calculated for this step.In addition,the mass gains of the aluminized samples were predicted using the partial pressures of gases in the pack and those adjacent to the samples surface.The predicted values are in good agreement wim the experimental measurements at 950 ℃ but are higher than those measured at 1025 and 1100 ℃.
文摘The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed in welds of titanium alloy laser welding based on the microscopic characteristics of the porosities. One is the metallurgical porosity with round and smooth inner wall, which results from the surface contamination. The other is the processing porosity with irregular and rough inner wall that displays the trace of the pool flowing, which results from the ruffle on the keyhole wall gathering together locally and closing down the gas in the keyhole into bubbles because of the keyhole fluctuating. The CO2 laser welding could break down easily the surface oxide film and produce little metallurgical porosity, but produces easily processing porosity when partial penetration or unstable-full penetration laser welding is conducted, which always occurs in the center of weld.
文摘Gam matitanium aluminide ( γ TiAl) alloys are emerging as a revolutionary engineeringmaterialsfor hightemperaturestructuralapplications. Onthebasisoftheinformation avail ablein the public domain, this paper discussesthe historical background,status and future prospect of gam maalloytechnologyintheareasofalloy development/ design,processdevelop ment, and applications.
文摘Reaction synthesis process has been used to develop γ titanium aluminide using elemental powders. Powder mixture of Ti-48 at. pct AI was prepared in ball mill and reaction synthesis was carried out in hot press with varying temperature and pressure. Titanium aluminide synthesized under high pressure and temperature resulted in better properties with respect to densification, homogenization response, mechanical properties and oxidation resistance as compared to that synthesized under low pressure and temperature. AI rich phases were observed in as-synthesized condition in all the experiments. However, some Ti rich phases were also found in high pressure-temperature synthesized samples. Density, hardness and tensile strength have been correlated with applied pressure through empirical relations. Variation in density with pressure is found to be logarithmic whereas hardness and tensile strength variation with pressure is polynomial.
文摘The relationship between the stress intensity factor K_1~* required for brittle crack initiation and propa- gation and the fractal dimension D_F of the fracture surface was derived,i.e, InK_1~*=(1/2)In2γE'+(1/2)In(d_f/L_0)(1-D_F) The real surface energy can be calculated based on the measured linear relation of the InK_1~* vs D_F, The equation is not only suitable for the overload fracture but also for the delayed fracture,e.g. hydrogen induced cracking (HIC) and stress corrosion cracking (SCC).The experiment results showed that the hydrogen induced delayed cracking occurred in the Ti-24AI-11 Nb alloy during dy- namic charging,and the threshold stress intensity factor was very low,i.e..K_(IH)/K_(IC)= 0.43.The ex- perimental relationship between the stress intensity factor K_1~* and D_F was consistent with the theo- retical equation.
基金supported by the National Natural Science Foundation of China(No.51831001)the Funds for Creative Research Groups of China(No.51921001)+1 种基金the Beijing Natural Sci-ence Foundation(No.2222092)the National Science and Tech-nology Major Project(No.J2019-Ⅵ-0003-0116).
文摘This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique.The results indicate that the mushy zone consists of unmeltedβdendrites and interdendritic liquid,whose formation can be attributed to the difference in melting point aroused by local heterogeneity in solutecontent.Theβdendrite is composed of numerous subgrains with various orientations.During quenching,theβdendrite transforms into Widmanstättenαvia a precipitation reaction,owing to the decreasing cooling rate caused by heat transfer from the surrounding liquid.Additionally,after quenching,the interdendritic liquid is transformed intoγplates.Within the singleβphase field and the lower part of the mushy zone,a massive transformation ofβtoγoccurs.Conversely,in theβ+αphase field,bothβandαphases are retained to ambient temperature.During the heating process,the transformation ofα→βgives rise to the formation ofβvariants,which affects the orientation ofβdendrites in the mushy zone.The growth kinematics of theα→βtransformation was elucidated,revealing the preferential growth directions of111and112forβvariants.Furthermore,this study presents an illustration of the formation process of the mushy zone and the microstructural evolution during the heating and quenching process.
基金Project (2011CB605500) supported by the National Basic Research Program of ChinaProject (FRF-MP-10-005B) supported by the Fundamental Research Funds for the Central Universities,China
文摘The double directional solidification（DS） technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys.Polysynthetically twinned（PST） crystals with lamellar boundaries parallel to or inclined 45o to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface.The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions.Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase.Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys,the tensile plastic elongations of the alloys are only close to 2%.
文摘The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%)alloy after subsequent quenching from 1380℃to 1000℃was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM)and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→βo+α2+γ→βo+γ+α2/γ→βo+γ+α2/γ+βo,sec,with the phase transformationα→βtemperature(Tβ)=1311℃,phase transformationγ→βtemperature of(Tγsolv)=1231℃,phase transformationα2→αorβo→βtemperature(Tα2→α/Tβo→β)=1168 C,eutectoid temperature(Teut)=1132℃and T(α2/γ→βo,sec)≈1120℃.In comparison with Ti-42 Al alloy,the Teut and Tγsolv are slightly increased while both the Tp is decreased obviously by 5%Mn addition.When quenched from the temperature of 1380-1260℃,the martensitic transformationβ→α'could occur to form the needlelike martensite structure inβarea.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃to 1380℃.When the temperature is below Tγsolv(1231℃),theγgrains would nucleate directly from theβphase.For the temperature slightly lower than T(eut)(1132℃),the dottedβ(o,sec)phases could nucleate in the lamellar colonies besides theγlamellae precipitated withinα2 phase.Finally,at room-temperature(RT),the alloy exhibits(po+α2+γ)triple phase with microstructure ofβo+lamellae+γ,of which the lamellar structure consists ofα2,γandβo,sec phases.The phase transformation mechanisms in this alloy,involvingβ→α',β→γ,α2→α2/γandα2→βo,sec were discussed.
基金This work is supported by the National Natural Science Foundation of China(51904205)Science and Technology Foundation of State Key Laboratory,China(6142909180205)+3 种基金China Postdoctoral Science Foundation(2018M641681)Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi Province,China(2019L0216)Shanxi Province Science and Technology Major Program,China(20181101008)Natural Science Foundation of Shanxi Province,China(201801D221346,201801D221221).
文摘Effects of cooling rate and 0.25 at.%TiB2 addition on solidification microstructure and mechanical properties of Ti−48Al−2Cr−2Nb alloys fabricated by the investment casting with different thicknesses were studied.The results show that with the cooling rate increasing from 37 to 2×102 K/s,the solidification path of the studied alloys is unchanged.The grain size of the matrix alloy is refined from 650 to 300μm,while the grain size of Ti−48Al−2Cr−2Nb−TiB2 is reduced from 550 to 80μm.The lamellar spacing of matrix alloy is reduced from 360 to 30 nm with increasing the cooling rate from 37 to 2×102 K/s,while TiB2 addition shows little refinement effect on the lamellar spacing.Ti−48Al−2Cr−2Nb−TiB2 sample under medium cooling rate(69 K/s)exhibits superior microhardness(HV 550)and ultimate tensile strength(570 MPa)among the studied alloys.The refined grain size,lamellar spacing and fine TiB2 particles could account for the favorable mechanical properties of the studied TiB2-containing alloy.The microstructure evolution was discussed in light of cooling rate,constitutional supercooling and borides addition.
文摘Solid state phase transformation characteristics of the body centred β（Ti） into the hexagonal closed packed α（Ti） in Ti45AI8Nb-（0, 0.3, 0.5 and 0.8 at. pct） B alloys were investigated by heat treatment to clarify γ/α2 lamellar microstructure refinement induced by B addition. Experimental results confirmed two kinds of boron-reduced grain refinement mechanisms through refining either/β phase then α phase （β-refinement） or α phase directly （α-refinement） to refine lamellar microstructure at room temperature; however, the role of α-refinement dominated the as-cast alloy. It was also found that during the lamellar microstructure refinement β-refinement the convoluted flake over β-refinement in Ti45Al8NbxB and plate-like borides along β grain boundaries assisted nucleation of α phase, and the particle-like borides nearβ grain boundaries impeded α phase growth.
基金Project(1343-74236000008)supported by Hunan Provincial Innovation Foundation for PostgraduateProject(2008AA03A233)supported by the High-tech Research and Development Program of ChinaProject(2007BAE07B05)supported by the National Science and Technology Planed Project of China
文摘Microstructures and deformation properties of Ti-46Al-(Cr,Nb,W,B)alloy consolidated by pseudo-HIP technology were investigated.The results show that the pseudo-HIP temperature has a significant effect on microstructures.When the sintering temperature is 1 100℃,the microstructure of as-pseudo-HIPped alloy is similar to that of the prealloyed powder and the interfaces of these powder particles are still discernible,but a nearγmicrostructure appears in particles.Increasing the pressing temperature to 1 200℃develops successfully a homogeneous and fine-grained duplex microstructure.A typically fully lamellar microstructure with residualβphase is developed at 1 300℃.The compact exhibits excellent deformation properties at elevated temperatures. When the compression temperature is higher than 1 100℃,high quality products without cracks can be obtained even if the engineering compression strain is up to 0.8 at strain rates of 10-2-10-3s-1.It can be established that the mechanical twinning and matrix deformation due to ordinary dislocation slip/climb contribute to the whole hot deformation.
基金Project (50771041) supported by the National Natural Science Foundation of ChinaProject (05-0350) supported by the New Century Excellent Talents in University, China
文摘A 0.7 mm-thick wavy γ-TiAl sheet with fully lamellar microstructure was fabricated by hot pressing Ti/Al alternate foils with heat treatment of 640 °C, 15 h＋850 °C, 35 h＋1350 °C, 2 h. The intermetallic compounds formed during heat treatments were identified by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. TiAl3 was the only observed phase at the Ti/Al interface when Al foils were not consumed. After being annealed at 850 °C for 35 h, the microstructure was composed of α-Ti, α2-Ti3Al, γ-TiAl and TiAl2. A fully lamellar microstructure formed after annealing at 1350 °C. Most of the angles between the lamellar interface and the sheet plane are below 30°. Using thinner starting foils is favorable to produce sheets with fine microstructure.
基金Funding from the Spanish Ministry of Science and Innovation through projects MAT2009-14547-C02-01 and MAT200914547-C02-02The Madrid Regional Government partially supported this project through the ESTRUMAT (Grant No.P2009/MAT-1585)
文摘A γ-TiAI intermetallic alloy, Ti-45Al-2Nb-2Mn （at.%）-0.8 vol.%TiB2, has been processed from gas atomized praalloyed powder by field assisted hot pressing （FAHP）. An initial analysis of the prealloyed powder helped on the understanding of the intermetallic sintering behavior. Atomized powder consisted of α metastable phase that transformed into α2＋γ equilibrium phases by thermal treating. Different powder particle microstructures were found, which influence the microstructure development of the FAHP T-TiAI material depending on the sintering temperature. Duplex, nearly lamellar and fully lamellar microstructures were obtained at the sintaring temperatures above 1000 ℃. Lower consolidation temperatures, below 1000 ℃, led to the formation of an AI rich phase at powder particle boundaries, which is deleterious to the mechanical properties. High compressive yield strength of 1050 MPa was observed in samples with FAHP duplex microstructures at room temperature. Whereas nearly lamellar and fully lamellar microstructures showed yield strength values of 655 and 626 MPa at room temperature and 440 and 425 MPa at 750 ℃, respectively, which are superior in comparison to similar alloys processed by other techniques. These excellent properties can be explained due to the different volume fractions of the α2 and γ phases and the refinement of the PM microstructures.
基金the financial support from the Singapore Ministry of Education Academic Research Funds(R-265–000–686–114 and MOE2018-T2–1–140)。
文摘The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100℃was systematically investigated in the temperature range of 750-900℃under an initial strain rate of 10^(-4)s^(-1).A bimodalγgraindistribution microstructure of Ti Al alloy sheet,with abundant nano-scale or sub-micronγlaths embedded insideβmatrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m=0.27 at 800℃,which is the lowest temperature of superplastic deformation for Ti Al alloys attained so far.The maximum elongation reaches~360%at 900℃with an initial strain rate of 2.0×10^(-4)s^(-1).To elucidate the softening mechanism of the disorderedβphase during superplastic deformation,the changes of phase composition were investigated up to 1000℃using in situ high-temperature X-ray diffraction(XRD)in this study.The results indicate thatβphase does not undergo the transformation from an ordered L2;structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of bothγandβdynamic recrystallization(DR)after tensile tests as characterized with electron backscatter diffraction(EBSD),the superplastic deformation mechanism can be explained by the combination of DR and grain boundary slipping(GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic deformation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic Ti Al alloy.
基金supported financially by the National Natural Science Foundation of China (Nos. 51604191, 51504163 and5 1501100)the Natural Science Foundation for Young Scientists of Shanxi Province, China (No. 201701D221075)+2 种基金the State Key Laboratory for Advanced Metal and Materials foundation (No. 2014-ZD06)the Special/Youth Foundation of Taiyuan University of Technology (Nos. 2015QN014, 2013T004 and 2013T003)the financial support of the Qualified Personnel Foundation of Taiyuan University of Technology (Nos. tyutrc201342a and tyutrc201343a)
文摘In order to understand the effect of Mo alloying on the microstructural evolution of TiAl alloy, the as-cast microstructure, heat treated microstructure characteristic, and hot compression microstructure evolution of Ti-44Al alloy have been studied in this work. The as-cast microstructure morphology changes from （γ＋α2） lamellar colony and β/β0＋γ mixture structure to β/β0 phase matrix widmannstatten structure, when Mo content increases from 2 at.% to 6 at.%. Affected by the relationship between β phase and α90℃phase, the angles between the lamellar orientation and the block β/β0 phase are roughly at 0℃, 45℃ and 90℃. Comparing with heat treatment microstructure, the hot compression microstructure contains less β/β0 phase, however, theβ/β0 phase containing 2Mo alloy and 3Mo alloy hot compressed at 1275 ℃ has the inverse tendency. In addition, （α2 ＋γ） colony is decomposed by the discontinuous transformation.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.
